The present invention is directed toward an optical communication system having reduced four-wave-mixing (FWM) and cross-phase modulation (XPM).
Optical signals transmitted in a fiber optic communication system typically constitute a series of pulses of digital information with a “1” bit represented by the presence of light at a relatively high intensity, and a “0” corresponding to a substantial reduction in optical intensity. In early fiber optic communication systems, the pulses of light were transmitted at a single wavelength. More recently, however, wavelength division multiplexing (WDM) has been explored as an approach for increasing the capacity of existing fiber optic networks. In a WDM system, plural optical signal channels are carried over a single transmission optical fiber with each channel being assigned a particular wavelength. The optical channels typically lie within narrow range about 1550 nm, the low loss band associated with silica based optical fibers. WDM systems further include a plurality of receivers, each detecting a respective channel by effectively filtering out the remaining channels.
In order to further increase capacity, high data rate time division multiplexed systems are also being developed. Such systems can be implemented with WDM equipment to provide WDM systems with enhanced capacity. For example, WDM systems carrying 16 channels, each of which being modulated at rates of 10 Gbit/s, are being developed which have a total capacity of 160 Gbit/s. At such high data rates, the optical pulses corresponding to the transmitted data are narrowed significantly. Accordingly, the optical power associated with each pulse is also increased in order to assure adequate detection of the transmitted signals. With increased power, nonlinear effects which are directly proportional to the peak power become more important. Conventional WDM systems transmit signals on non-dispersion shifted fibers (NDSF) which are quite insensitive towards non-linear effects. More recent WDM systems use non-zero dispersion shifted fibers (NZDSF) and dispersion shifted fibers (DSF) which by design have low dispersion. Examples of NZDSF fibers are Corning LS®, Corning LEAF®, and Lucent TW®. These transmission fibers are primarily used to reduce the dispersion compensation requirements of the entire system. However, the low fiber dispersion makes them sensitive to nonlinear effects in particular FWM and XPM (explained below) causing errors in transmission. Note that nonlinear effects will also become important in NDSF systems when the power per channel is increased as would be required by systems having high span losses or long propagation distances or when the channel spacing is decreased.
One type of nonlinearity, known as cross phase modulation (XPM), is caused by the optical intensity of one signal modulating the phase of another optical signal due to intensity dependent refractive index variations of the fiber. The phase modulation can impose a frequency chirp that redistributes the signal frequency spectrum such that lower frequencies are shifted toward the leading edge of an optical signal pulse and higher frequencies are shifted toward the trailing edge. These changes in frequency distribution coupled with the chromatic dispersion of transmission fiber can cause intensity modulation and degrade the transmitted optical signals. For a fixed amount of power per channel, XPM is inversely proportional to the frequency separation between channels, the dispersion of the transmission fiber (i.e., the “local dispersion”), and the number of channels present.
Most WDM systems having transmission distances greater than a few hundred kilometers require some form of dispersion compensation to offset the chromatic dispersion of the transmission fiber in which various spectral components of an optical signal pulse propagate through the fiber at different speeds and distort the pulse shape. The dispersion compensating devices, one of which can be a segment of dispersion compensating fiber (DCF), can be placed at appropriate points in a WDM link between the transmitter and receiver. In addition, the magnitude of dispersion compensation is known to significantly effect, by either decreasing or increasing, the level of XPM experienced by a particular channel in the system (see, for example, T. K. Chiang, N. Nobuyuki, M. E. Marhic, and L. G. Kazovsky,“Cross-Phase Modulation in Fiber Links with Multiple Optical Amplifiers and Dispersion Compensator,” Journal of Lightwave Technology, vol. 14, pp. 249-259, 1996).
Although, in theory, dispersion compensation can be used to partly offset XPM and the resultant intensity modulation, in practice, several problems can be encountered. First, to reach an optimal dispersion compensation which significantly reduces the effects of XPM, the dispersion compensating element should have predetermined precise values depending on parameters unique to each system. Most commercially available dispersion compensating devices, e.g., DCF, do not have one unique dispersion value, but are warrantied to have dispersion values within a nominal range.
Second, the chromatic dispersion of the transmission fiber is not precisely known, but is merely specified to be within a particular range of values, as well, and the variation of its dispersion value may be more or less than that of the dispersion compensating element. Accordingly, in light of the above, commercially available dispersion compensating elements cannot be used to offset the effect of XPM unless time consuming dispersion measurements are undertaken during installation.
Moreover, in addition to uncertainties in the precise knowledge of the amount of dispersion as discussed above, WDM systems may contain other dispersion elements like in-fiber Bragg gratings and interference filters which may also have an inherent amount of dispersion that varies due to design or manufacturing limitations. Such elements are typically used for channel selection in a multichannel systems, and any variation in their dispersion may cause the predetermined optimal dispersion (i.e., the “dispersion map”) of certain channels to change so that a sub-optimal amount of XPM reduction is achieved.
In addition to XPM, intensity dependent fiber refractive index variations can cause co-propogating optical signals at different wavelengths to interact to produce additional optical frequencies through four wave mixing (FWM). In particular, for two channels at frequencies fi and fj (fj>fi) such that fj−fi=Δf is the frequency separation, the process of FWM adds new optical frequencies or “mixing products” in the spectrum at frequencies fi−Δf and fj+Δf. In a multichannel system, if all channels are equally spaced by Δf, the mixing products can fall on the same spectral locations as the channels neighboring fi and fj, contributing as noise to those neighboring channels and degrading their performance. The magnitude and efficiencies of FWM for a fixed amount of power per channel is also inversely proportional to the frequency separation of the channels, the local dispersion of transmission fiber, and the number of channels present.
In order to reduce non-linear effects, such as FWM, optical frequencies are assigned unique values in accordance with a channel plan. Channel plans have been proposed whereby the channels are equally spaced, but relatively far apart from one another in order to minimize these nonlinearities. Such equally spaced channel plans require a significant spectral range or bandwidth which can exceed the low loss transmission band for silica based optical fibers. Equally spaced channel plans having large channel separations are thus usually unsuitable for high channel count WDM systems.
An alternative channel plan including non-uniform or unequal channel spacings is described, for example, in U.S. Pat. No. 5,546,210. This scheme, however, typically also requires a relatively large bandwidth to accommodate each of the channels, although not as much as the equally spaced channel plans described above. The amount of bandwidth required is quantified with a parameter referred to as a bandwidth expansion factor (BEF), which is defined as the bandwidth ratio between an unequally spaced channel plan and an equally spaced channel plan for a given number of channel. An exemplary BEF of 1.8 is described in the above patent, but a lower BEF is desirable.
Moreover, the International Telecommunications Union (ITU) has standardized a frequency “grid”, which WDM systems are expected to conform to. The grid consists of equally spaced frequency designations 50 GHz apart. Most unequally spaced channel schemes require non-standard frequencies or channels that do not conform to the ITU grid, and may have limited commercial acceptance.